Project Summary Hybridization Chain Reaction: In Situ Amplification for Biological Imaging Each cell in a multi-cellular organism contains the same genome, yet the regulatory circuits encoded within this genome implement a developmental program yielding significant spatial heterogeneity and complexity. In situ hybridization methods are an essential tool for elucidating developmental and pathological processes, enabling imaging of mRNA expression in a morphological context from sub-cellular to organismal length scales. Due to variability between specimens, accurate mapping of spatial relationships between the regulatory loci of different genes requires multiplexed experiments in which multiple mRNAs are imaged in a single biological sample. With current in situ hybridization approaches, it is challenging to simultaneously detect the expression of multiple target mRNAs within intact vertebrate embryos. This shortcoming is a significant impediment to the study of interacting regulatory elements in systems most relevant to human development and disease. Here, we draw on concepts from the field of nucleic acid nanotechnology to design and validate in situ am- plifiers based on the mechanism of hybridization chain reaction (HCR). Using this approach, RNA probes com- plementary to mRNA targets trigger chain reactions in which fluorophore-labeled RNA hairpins self-assemble into tethered fluorescent amplification polymers. During the first funding period, we engineered orthogonal HCR amplifiers that operate independently in the same sample at the same time. Robust performance was achieved when imaging five target mRNAs simultaneously in fixed whole-mount and cross-sectioned zebrafish embryos. Moreover, HCR amplifiers exhibited excellent sample penetration, high signal-to-background, and sharp signal localization. During the second funding period, we will extend the core HCR in situ amplification technology to pursue unprecedented quantitative imaging goals in vertebrate embryos, to diversify the classes of targets and organisms for which the technology is validated and optimized, and to engineer next-generation HCR in situ amplifiers with improved properties. Our major goals are: Accurate and precise relative quantitation of mRNA abundance across whole-embryo images. Sub-cellular imaging of single mRNA transcripts with quantitative yield in whole-mount zebrafish embryos. Multiplexed mapping of miRNAs and alternatively spliced mRNAs with high signal-to-background in whole- mount zebrafish embryos. Generalizing HCR in situ amplification for use in diverse organisms. Engineering next-generation HCR in situ amplifiers with dramatically improved gain, uniformity, speed, and cost. Realization of these goals would have a broad impact on research in the biological sciences, providing an un- precedented combination of multiplexing, quantitation, sensitivity, and resolution for the study of interacting RNA regulatory elements within intact vertebrate embryos and other diverse biological samples.